The usual photovoltaic cell is fabricated of silicon, germanium, gallium arsenide or other such semiconductor material in which a p-n junction is formed and through which light is projected to cause electron carriers to cross the p-n junction. Electrons in such materials are usually not free to move from atom to atom within the crystal, but light striking such crystals provides the energy needed to free some electrons from their bound condition. Free electrons cross the p-n junction more easily in one direction than in the other giving one side of the junction a negative charge and, therefore, a negative voltage with respect to the other side. The efficiencies of such crystals range from 7 to 14 percent with a maximum theoretical efficiency in the range of 20 percent.
A major cause of the low efficiencies of photovoltaic cells is so-called junction loss which results from the flow of carriers (electrons or holes) back across the junction. Back flow can be minimized by increasing the band gap of the junction and efficiency can thereby be increased for selected wavelength ranges, but efficiency in converting solar energy will still be low because only a limited portion of the solar energy spectrum can be utilized. Photovoltaic cells employing graded band gaps have been proposed to better utilize the solar energy spectrum. Also, cells with graded impurity levels have been proposed to produce an internal field in the bulk material to reduce junction loss. Nevertheless, no structure has been developed with the specific materials required to successfully implement such concepts.
A second problem in utilizing many inexpensive semiconductor materials for solar energy conversion is the difficulty of obtaining high carrier collection efficiencies. This results because it is often difficult to manufacture these materials with sufficient quality to obtain long carrier lifetimes or high carrier mobilities.